In order to manufacture a steel plate having little deformation, such as curvature deformation, by controlling TMCP (Thermo Mechanical Control Process) or internal stress for improving strength and toughness of a steel plate with microstructure as a fine structure of which the crystal grain diameter is 1 μm or so, the start temperature of controlled rolling, finishing temperature, cooling start temperature of accelerated cooling, and cooling stop temperature need to be strictly managed, and accordingly, a measurement method for measuring the temperature of a steel plate with high precision, or cooling equipment of which an innovative placement of a thermometer has been made, has been employed as a manufacturing technique or equipment.
For example, with Japanese Examined Patent Application Publication No. H7-41303, description is made wherein, with regard to cooling control equipment of hot-rolled steel plate, in order to prevent defective shape due to curvature deformation in the plate-width direction at the time of cooling, steel plate temperature is measured, cooling water flow rate from each nozzle disposed above and below of the cooling equipment, cooling start, and cooling end are strictly controlled, and an optical fiber thermometer is disposed on the downstream side of a finishing mill, and on the upstream side, downstream side and the inside of the cooling equipment. Note that hereafter, unless otherwise stated, the upstream side and the downstream side mean that as to the advancing direction (also referred to as threading direction) of a steel plate manufacturing line, placement is made on the upstream side and downstream side, respectively.
With Japanese Unexamined Patent Application Publication No. H10-5868, description is made wherein, with regard to a shape control method of a steel plate subjected to controlled cooling, a shape after cooling to normal temperature of a steel plate subjected to accelerated cooling is estimated from a shape immediately after cooling, and the temperature history of a steel plate, a thermometer for ensuring the shape of the following plate and measuring temperature of the upper and lower surfaces of a steel plate is disposed within the accelerated cooling equipment, and a temperature distribution meter (thermo-tracer) is disposed on the upper surface of the steel plate, and a thermometer is disposed on the upper surface of the steel plate immediately after the accelerated cooling equipment.
With Japanese Unexamined Patent Application Publication No. 2001-300627, description is made wherein, with regard to a steel plate cooling method, in order to realize improvement in the flatness of a steel plate shape, and uniformity of quality, controlled cooling is performed by suppressing scale thickness variance to 10 μm or less by descaling or application of a surface coating film after finishing rolling, and a radiation thermometer is disposed on the upstream side of the controlled cooling equipment as a surface thermometer.
Also, with Japanese Unexamined Patent Application Publication No. S52-117857, a method is described wherein temperature distributions in the rolling direction and the plate width direction are measured only on the downstream side of the final finishing rolling, temperature difference as to a required value is obtained, and with this as an index, a portion deviating from this temperature difference index is discarded.
Incidentally, recently, with regard to heat-treated material (particularly, high nickel steel of which the quality sensitivity as to deviation of manufacturing conditions is high), manufacturing has been made by the direct quenching method, and there are more applications of TMCP for streamlining. Also, heretofore, the quality of a steel plate has passed by assuring a part thereof such as a central portion in the width direction, but the number of user requests for quality assurance of the whole area of a product plate has increased. Also, the strictness of requests thereof has been increasing year by year.
Also, with a line pipe base plate, shipbuilding material, and so forth, a nonconventional request for assuring strength variation within a steel plate to be a particular value or less has also emerged.
On the other hand, with regard to a steel plate, during the manufacturing process thereof, the steel plate is passed through a continuous reheating furnace, a roughing mill, a finishing mill, a descaling device, an accelerated cooling equipment, and so forth, and accordingly, temperature distribution on the plate surface or in the plate thickness direction readily becomes uneven, and as a result, the quality also readily becomes uneven. Accordingly, in many conventional cases, for example, quality determination has been made wherein according to a radiation thermometer attached above a transportation line, temperature of the central portion in the plate-width direction of the steel plate is continuously measured in the rolling direction, and in the event that the temperature measurement only on the central portion thereof is included in a allowable temperature range, the quality determination of the whole steel plate is passed, and in the event of deviating from the allowable temperature range, the quality determination fails.
However, with quality determination using temperature measurement results by the above radiation thermometer, a particular portion (e.g., temperature of the central portion in the width direction of a steel plate) is measured by a radiation thermometer attached on the upper surface of the steel plate in the center in the transportation line width direction, the measurement temperature thereof and each of the management temperature ranges are compared, the quality determination is carried out only for the particular portion, and accordingly, the quality assurance of the full length and full width (whole area) is insufficient.
However, the quality of a steel plate is changed by the structure thereof being changed due to conditions such as controlled rolling, accelerated cooling, and so forth, and particularly, it has been known that of the conditions, the temperature of a steel plate has a great impact on the quality thereof, and with a conventional method described in the above Japanese Unexamined Patent Application Publication No. S52-117857, even when a temperature history of a steel plate during rolling receives is changed, temperature is evaluated only on the downstream side of the final finishing rolling, and accordingly, if determination is made with temperature difference on the downstream side of this finishing rolling, there are many cases deviating from the quality originally assured regarding partially collected products from the large plate of a steel plate, and sufficient quality assurance cannot be realized, which causes major concerns. Also, as described in an embodiment of Japanese Examined Patent Application Publication No. H7-41303, the temperature of any one surface of the upper surface of a steel plate, or the lower surface of a steel plate is measured, and accordingly, with regard to an extremely-thick steel plate having great plate thickness, there are cases where evaluation cannot be performed regarding the difference between the temperature of the upper surface of the steel plate, and the temperature of the lower surface of the steel plate, demand has also been strong regarding quality assurance in the plate thickness direction.
Also, a steel plate such as a pipe plate is manufactured in large quantities by the same manufacturing conditions, and accordingly, with a conventional method for determining various types of quality with test specimens collected from each position of the front end, middle, and tail end, in the rolling direction, there are cases where it takes several days after the test specimens of a steel plate are collected, and if determination were to become impossible, all steel plates manufactured during the period thereof might fail, resulting in massive defective plates.
Also, quality variance equivalent to temperature variance is taken into consideration, and with a quality design, in order to provide a margin as to the lower limit specifications of material property (e.g., mechanical property), extra alloying elements need to be added, and manufacturing cost becomes expensive. Further, the target temperature range of acceptance/rejection is narrowed, and accordingly, there is a problem such that a material having strict quality specifications cannot be manufactured with an accelerated cooling process after rolling.
Therefore, the present invention provides a steel plate online quality assurance system capable of rapidly determining the quality of a steel plate subjected to finishing rolling and accelerated cooling to assure the quality thereof.
Also, a steel plate quality assurance system is provided for accurately measuring steel plate temperature on a manufacturing line to evaluate uniformity of quality by predicting quality within the plate surface of a steel plate from the obtained temperature distribution.
Also, steel plate quality assurance equipment is provided that is capable of operating control for improving quality uniformity in the plate thickness direction of a steel plate and within the steel plate surface by suitably disposing a thermometer for measuring steel plate temperature in a steel plate manufacturing line.
Also, a steel plate quality assurance method is provided wherein steel plate temperature is measured in the manufacturing line, the temperature of the whole area of the upper and lower surfaces of a steel plate is calculated from the measured steel plate temperature, and based on the temperature of the calculated whole area, quality within the plate surface of a steel plate is evaluated, and the quality thereof is determined and assured.